There is an increasing interest in the development of a compact diode laser source in the 400-500 nm range for advanced applications, such as the enhancement of optical disk technology through improved data storage, retrieval storage density, and data capture rates.
There are several possible approaches to a prospective short-wavelength diode laser source. The traditional diode materials such as the ternary and quaternary compounds of In, Ga, As, Al, P, and Sb do not have a direct energy gap high enough to produce a short wavelength laser. A number of other laser materials have been studied for development of diode laser sources in the blue wavelength region. These materials include II-VI semiconductors, quantum well materials, and other wide band gap semiconductors such as cadmium sulfide (CdS). Although the blue quantum well structure shortens the lasing wavelength, it requires a cryogenic condition for lasing. There also has been investigation of II-VI wide-gap superlattices with the goal of achieving diode emission in the blue-green region of the spectrum, but these materials have only produced optically pumped lasers requiring cooling at liquid nitrogen temperature. Other semiconductors that are known to lase in the 450-500 nm range are CdS, ZnSe, ZnCdS, and CdSeS. Lasers of these materials require either optical or electron beam pumping for their operation.
In view of the impracticality of a direct diode laser source, attractive alternatives involve frequency conversion of available diode laser sources, either by frequency doubling or parametric up-conversion. The traditional frequency conversion techniques utilize phase matching of input beams and harmonic waves in second order optical crystals such as potassium dihydrogen phosphate (KDP and KD*P), LiNbO.sub.3 and LiIO.sub.3. However, because of the relatively low values of second order susceptibility of these crystals, and the low beam intensity of a diode laser, an exceptionally lone single crystal is required to achieve appreciable lower conversion to the second harmonic tensor. Such large crystal dimensions preclude the design and fabrication of a compact and ruggedized optical recording system. In addition, the provision of large inorganic crystals is difficult and costly.
In general, classical phase matching (e.g., via angle or thermal tuning) requires a certain combination of intrinsic birefringence and dispersion of refractive indices. New small molecular weight crystalline organic nonlinear optical materials with high second harmonic susceptibility have been reported in literature such as ACS Symposium, Series No. 233, pages 1-26, 1983 by Garito et al. These organic materials usually possess high intrinsic birefringence and positive dispersion so that phase matching can be achieved with a single crystal. Even if phase matching can be achieved with the new types of organic materials having high nonlinear optical susceptibility, the low beam power of a diode laser significantly limits the lower conversion efficiency. The high birefringence of the organic materials also lowers the conversion efficiency because of beam walk-off
An alternative means to achieve phase matched conditions is the use of dispersion properties for different modes in a waveguide. Since the energy is confined laterally to a narrowly constricted space, a high light intensity is possible with a relatively low power source. In this approach, one usually excites a lower order mode of the fundamental beam and the second harmonic generated propagates in a higher order mode. If the waveguide geometry and refractive indices of the guiding region and cladding region are such that: EQU .DELTA..beta.=.beta..sub.n (.omega..sub.3)-.beta..sub.m (.omega..sub.2)-.beta..sub.1 (.omega..sub.1)=0 (1)
then the phase matching condition is established. Here, .beta..sub.1 is the propagation constant of the i-th mode. The conversion efficiency is generally quadratically dependent on the overlap integral between the two modes: EQU F=.intg.E.sub.n (.omega..sub.3, z)E.sub.m (.omega..sub.2, z) E.sub.1 (.omega.1, z)dz
where E is the normalized electric field of the mode across the waveguide. In general, the overlap between the waveguide modes is limited, and the value of the overlap integral is also small. This approach has been utilized for phase matching in waveguides derived from organic materials, as reported in Optics Commun., 47, 347 (1983) by Hewig et al. However, the level of second harmonic conversion efficiency is too low for any practical frequency doubling application.
Of interest with respect to the present invention is literature relating to spatially periodic nonlinear structures for frequency conversion of electromagnetic energy. The pertinent literature includes IEEE J. of Quantum Elect., QE-9 (No. 1), 9 (1973) by Tang et al; Appl. Phys. Lett, 26, 375(1975) by Levine et al; Appl. Phys. Lett, 37(7), 607(1980) by Feng et al; and U.S. Pat. Nos. 3,384,433; 3,407,309; 3,688,124; 3,842,289; 3,935,472; and 4,054,362.
The thin film waveguides with a periodically modulated nonlinear optical coefficient as described in the literature are either inorganic optical substrates with material fabrication disadvantages, or they are organic materials which are in the liquid phase, such as a liquid crystal medium or a thin film of nitrobenzene which require a continuously applied external DC electric field.
Of particular interest with respect to the present invention is literature relating to the dispersive properties of a thin film optical waveguide for TE and TM modes, as expressed in analytic terms defining the variation of the effective refractive index with respect to one or more physical parameters in the waveguiding medium. The pertinent literature includes J. Appl. Phys., 49(9), 4945(1978) by Uesugi et al; Appl. Phys. Lett , 36(3), 178(1980) by Uesugi; Nonlinear Optics: Proceedings Of The International School Of Materials Science And Technology, Erice, Sicily, July 1-14, 1985 (Springer-Verlag), pages 31-65 by Stegeman et al; Integrated Optics, Volume 48, pages 146-151 by Ostrowsky (Springer-Verlag, 1985) Integrated Optics, Volume 48, pages 196-201 by Bava et al (Springer-Verlag, 1985); and Appl. Opt., 25(12), 1977 (1986) by Hewak et al.
There is continuing interest in the development of compact and efficient nonlinear optical devices, such as frequency converters and parametric oscillators and amplifiers.
Accordingly, it is an object of this invention to provide an organic nonlinear optical waveguide device which is adapted to double the frequency of an input laser beam.
It is another object of this invention to provide a polymeric nonlinear optical waveguide device with a spatial periodic structure for quasi-phase matching of propagating wave tensors, and with a refractive index tuning means for efficient phase mathcing of fundamental and generated second harmonic light beams.
Other objects and advantages of the present invention shall become apparent from the accompanying description and drawings.